1. FIELD OF THE INVENTION
The present invention relates to a high density magnetic recording medium.
2. DESCRIPTION OF THE PRIOR ART
At present, as magnetic recording media, those obtained by kneading ferromagnetic particles with a binder vehicle and coating on a non-magnetic substrate are generally employed. However, with the expansion of uses of magnetic recording, even further higher density is required.
As the characteristics which make high density recording possible, the following are contemplated:
(1) the coercive force of the ferromagnetic particles is high;
(2) The Is (saturation magnetization intensity) of the ferromagnetic particles is high;
(3) The particle size of the ferromagnetic particles is small;
(4) The ferromagnetic particles are uniformly dispersed in the binder vehicle;
(5) The magnetic coat is smooth and there is no spacing loss; and so forth.
In respect to the requirements (1), (2), and (3), the production of ultrafine particles has been made possible by the improvement of needle-shaped iron oxides, or the advent of cobalt-adsorbed needle-shaped iron oxides and recent needle-shaped metallic iron, or the innovation of production techniques of ferromagnetic particles.
The method for dispersing ferromagnetic particles in a binder vehicle requires the use of a highly efficient dispersing machine, and the improvement of the binder plays an important role. However, with respect to such ultrafine particles, satisfactory levels of dispersion have not yet been achieved.
The most common composition of a binder vehicle used for a magnetic recording medium is a mixture of a vinyl chloride-vinyl acetate-vinyl alcohol copolymer and a polyurethane elastomer or a butadiene-acrylonitrile copolymer. Examples of the vinyl chloride-vinyl acetate-vinyl alcohol copolymer include VAGH produced by Union Carbide Co., S-lec A produced by Sekisui Chemical Co., Vinylite #1000G produced by Electro Chemical Industy Co., etc., and the proportion of the vinyl chloride, vinyl acetate and vinyl alcohol is approximately 91:3:6.
However, such a copolymer has been inadequate with respect to dispersibility, the surface gloss on surface calender processing, and the adhesion to a substrate.
In order to solve these problems, it has now been discovered that a magnetic coat having excellent properties may be obtained by changing the compositional ratio of the vinyl chloride-vinyl acetate-vinyl alcohol copolymer. That is, it has now been discovered that by changing the contents of vinyl acetate and vinyl alcohol in the copolymer, the dispersibility or the surface gloss on calender processing, and the adhesion to a substrate can be improved.